Summary

Robotic Assisted Unicompartmental Knee Arthroplasty Restores Native Joint line More accurately

Abstract

Does Robotic Assisted Unicompartmental Knee Arthroplasty Restore Native Joint line
More accurately than with Conventional Instruments?

Introduction

Alteration of the joint line position following joint replacement leads to disastrous
effects on the long-term outcomes for the patient. The semi-autonomous hand held robotic
device has increased the accuracy of component placement as planned pre-operatively. We
hypothesise that this accuracy also contributes to precise restoration of the jointline, thus
avoiding complications.
A randomised prospective cohort study of 100 patients undergoing unicompartmental knee
arthroplasty (UKA) for antero-medial osteoarthritis was initiated in March 2017 in a speciality
high volume arthroplasty unit at a tertiary care centre. All patients were clinically and
radiologically assessed with Anteroposterior scanograms, Lateral and skyline views. Limb
alignment and preoperative radiological joint line assessment was performed by 2 fellowship
trainees using Weber’s method. The patients were randomised into 2 groups and UKA was
performed either with handheld semi-autonomous robot (Group A) or by conventional UKA
instrumentation (Group B). All procedures were performed by a senior experienced surgeon. All
cases in Group A received a fixed bearing UKA while the Group B cases were implanted with a
Mobile bearing design. 
Both groups were matched demographically with the average age of patients in Groups A and B 
being 57.7 yrs and 59.9 yrs respectively. Both groups had a higher female representation (Group
A 70% and Group B 68.5%). 
All radiological measurements were done using ImageJ (NIH, Bethesda MD). The height of the
joint line was calculated by the method described by Weber, wherein an angle was defined by a
line joining the most distal points of both femoral condyles and a second line tangent to the
lateral cortex . A successful joint line restoration was defined when the replaced
femoral component was measured within two millimetres from native joint line. The 2 millimeter
value is to compensate for the average thickness of articulating cartilage which is radiolucent. 

Results

The femoral component distal positioning was higher in Group B using the conventional
instrumentation group as compared to the Group A with robotic assisted UKA. This difference in
position was statistically significant. In Group A, mean difference between the pre-operative and
post-operative joint line was 1.6± 0.49 (range 0.8mm to 2.4mm) while in Group B it was
measured at 2.47± 0.51 (range 1.6mm to 3.9mm) (p < 0.005).  A higher percentage of patients
(64%) achieved a femoral component position = two millimeters from the joint line in Group A
while only 18% from Group B achieved the target.

Conclusion

The meticulous planning and attention to ligament balancing in the Robotic-
assisted UKA not only increases the surgical accuracy in implant positioning but also achieves
better native joint line restoration and balancing compared to conventional UKA technique. The
cohort needs to be followed for a longer duration to validate the longevity and survivorship.

Key words: Robotic UKA; Conventional UKA; joint line restoration
Abbreviations; UKA – UNICONDYLAR KNEE ARTHROPLASTY