Unicompartmental knee arthroplasty (UKA) allows replacement of a single compartment in patients with limited disease. However, UKA is technically challenging and relies on accurate component positioning and restoration of natural knee kinematics. This study examined the accuracy of dynamic, real-time ligament balancing using a robotic-assisted UKA system. Surgical data obtained from the computer system were prospectively collected from 51 patients (52 knees) undergoing robotic-assisted medial UKA by a single surgeon. Dynamic ligament balancing of the knee was obtained under valgus stress prior to component implantation and then compared to final ligament balance with the components in place. Ligament balancing was accurate up to 0.53 mm compared to the preoperative plan, with 83% of cases within 1 mm at 0°, 30°, 60°, 90°, and 110° of flexion. Ligamentous laxity of
Unicompartmental knee arthroplasty (UKA) has seen resurgence in the past decade with approximately 51,300 cases performed in 2009 and an estimated growth of 32.5% annually [
UKA allows for minimal disruption of the patient’s native anatomy and is intended to restore the normal height of the affected compartment to produce normal ligament tension during the flexion-extension cycle. The success of UKA relies on proper soft-tissue tensioning to obtain a balanced flexion-extension gap and varus-valgus stability [
Robotic-assisted UKA allows for improved component positioning [
While the surgical technique using a robotic-assisted UKA system has been described elsewhere in detail [
Ligament balancing was measured throughout various angles during the flexion-extension cycle relative to tibia and mechanical axis. (a) The colored dots represent measurements during femoral range of motion. (b) Intraoperative screenshot of the robotic system showing ligament balance at 0°, 30°, 60°, 90°, and 110° of flexion before resection, with the trial component in place, and after implantation.
The values obtained during the range of motion with valgus stress serve as the intraoperative balance plan for ligamentous tensioning. Using the computer system, component position or size can be altered, and the resulting changes in predicted ligament balance can be observed in real-time. If there is predicted laxity, component size and position can be changed to increase tightness, thereby programming the robot to alter bone cuts based on the preoperative CT scans and intraoperative findings. After the bone resections have been made using the robotic arm, the trial components are inserted and ligamentous tension is compared to the intraoperative balance plan. If proper balance is achieved with the trial components in place, the final components are inserted and cemented, and final ligamentous balance is obtained during range of motion.
The intraoperative data from 51 consecutive patients (52 knees) who underwent robotic-assisted UKA (MAKOplasty, MAKO Surgical Corp.) of the medial compartment by a single surgeon (RHJ) were prospectively collected over a 6-month period. All patients received a fixed-bearing UKA with an onlay cemented tibial component and cemented femoral component. Following registration of the robotic system and prior to incision, the intraoperative balance plan for ligament tensioning was obtained under valgus stress. After implantation of the final components, dynamic measurements were repeated without valgus stress. Data was stored on the computer system (Figure
The mean age of patients in this study was 67 years (range, 50–90 years) with a mean body mass index of 31.4 kg/m2 (range, 21.5–43.8 kg/m2). The surgical indication in all patients was isolated osteoarthritis of the medial compartment of the knee. Intraoperative measurements under valgus stress before component implantation revealed that ligamentous balance significantly changed during the flexion-extension cycle (Figure
Analysis of ligament balance at various degrees of knee flexion. The intraoperative balance plan was similar measurements obtained after component implantation at 0°, 60°, 90°, and 110°. At 30°, ligament balance was relatively loose and surgically corrected, revealing a significant difference (
Overall, the variation in ligament tensioning between the intraoperative balance plan and measurements after component implantation was less than 1 mm in 83% of the cases (Table
Comparison of the intra-operative balance plan and ligament balance measurements following component implantation. Data is expressed as mean ± standard error of the mean in millimeters.
Flexion angle | Balance plan | After implantation | Change in balance |
|
---|---|---|---|---|
0° | 0.34 ± 0.12 | 0.08 ± 0.18 | −0.26 ± 0.17 |
|
30° | 1.31 ± 0.13 | 0.78 ± 0.17 | −0.53 ± 0.18 |
|
60° | −0.28 ± 0.11 | −0.33 ± 0.14 | −0.04 ± 0.15 |
|
90° | −0.49 ± 0.12 | −0.32 ± 0.13 | 0.16 ± 0.13 |
|
110° | 0.03 ± 0.16 | −0.07 ± 0.19 | −0.10 ± 0.14 |
|
At 0° (a), 60° (c), 90° (d), and 110° (e), ligament balance between 1 mm and −1 mm was achieved in 81% to 93% of cases. At 30° (b), 76% of cases were balanced between 1 mm and −1 mm due to a necessary increase in ligament tightness.
Change at 0°of flexion
Change at 30°of flexion
Change at 60°of flexion
Change at 90°of flexion
Change at 110°of flexion
Successful outcomes of UKA rely on the restoration of normal knee kinematics and muscle lever arms of the knee joint. Therefore, restoration of proper ligamentous length and tension is a vital component of the UKA surgical technique. Using a robotic-assisted UKA system, we showed that real-time, dynamic ligament balancing reproduced planned ligamentous balance and, when appropriate, was able to increase ligament tightness when there was relative preoperative laxity.
Whiteside pointed out that proper ligament balance in combination with component alignment and fixation is vital for the success of UKA [
During conventional UKA, soft-tissue balance is assessed with the trial components in place and with subjective varus-valgus stress testing, commonly at 0° and 90° [
Specifically, fixed-bearing tibial components, such as the implants used in this study, rely on proper soft-tissue tensioning. There is low conformity between the femoral and tibial components with low contact areas allowing for unconstrained movements between the femur and tibia controlled only by the ligamentous apparatus [
There have been numerous advances in UKA instrumentation and cement or cementless fixation techniques that have led to an increase in the survivorship of UKA in the past decade [
A major limitation of this study is the lack of clinical or functional outcomes in this patient cohort; the study was intended to assess the accuracy of ligament tensioning only based upon the intraoperative balance plan. There are currently no studies available on the clinical outcomes of robotic-assisted UKA due to the novelty of the device. Certainly, long-term studies on the outcomes of the robotic-assisted device compared to manual UKA are needed to delineate a possible advantage of the robot in light of the financial investment. However, based on the technical demands of UKA, we believe that improved component positioning and alignment in combination with dynamic, real-time assessment of ligament balance offered by the robotic-assisted system may improve outcomes.
To our knowledge, this is the first study assessing real-time dynamic ligament balancing with a robotic-assisted system for UKA. We conclude from our findings that robotic-assisted UKA can accurately and precisely reproduce intraoperatively planned ligamentous balance using real-time, dynamic measurements. In combination with high accuracy of component placement, robotic-assisted systems may improve functional outcomes and survivorship of UKA patients; however, further investigations into the benefits of robotic systems for UKA are needed.
The authors J. F. Plate, A. Mofidi, S. Mannava, B. P. Smith, and J. E. Lang report no conflict of interests. The authors R. H. Jinnah, G. G. Poehling, and M. A. Conditt have received financial support from MAKO Surgical Corp., Fort Lauderdale, FL, USA. R. H. Jinnah and G. G. Poehling have received payment as consultants and are stock holders. M. A. Conditt receives compensation as Senior Director of Clinical Research. All authors certify that this investigation was performed in conformity with ethical principles of research. Institutional Review Board approval was obtained prior to the study.