Unity Knee: A modern total knee design based on the importance of understanding the role of joint line restoration on mid-flexion stability

Total knee replacement systems have evolved significantly over the past few decades, leading to an overall reduction in revision rates. Despite the advancements in materials and surgical technique, a large portion of patients remain dissatisfied1.Understanding this, Corin’s development team aimed to create a new knee implant system to address this challenge.


Understanding the effect of joint line restoration on knee stability


Unity Knee was designed by a global surgeon team with the aim to improve patient satisfaction after total knee replacement (TKR). The development team started by asking what happens to the overall knee kinematics depending on varied resections of the distal femur.

Firstly, this question was explored by the team as part of a cadaveric study. In the study, TKR procedures were carried out on cadaveric knees after three different levels of distal resection, starting at a ‘0mm’ resection to match the joint line. Coronal stress was then applied to each knee to assess stability at five flexion angles. The joint line was then raised by resecting an additional 2mm off the distal and posterior femur, with a corresponding increase in the insert thickness. The same coronal stability test was then repeated before raising the joint line another 2mm and conducting the stability testing a third and final time.



This video demonstrates the coronal laxity test carried out on the knee:

TKA0: Standard distal resection to match implant thickness

TKA2: Additional 2mm distal resection (raising the joint line)

TKA4: Additional 4mm distal resection (raising the joint line)


The results demonstrate that despite a balanced knee in extension and 90 degrees of flexion regardless of the amount of bone resected, the mid flexion laxity increased by 50% and 100% after further 2mm and 4mm resection, respectively2. Elevation of the joint line, therefore, appears to have a significant impact on mid-flexion stability.


So how do we preserve the joint line?


Unity Knee was designed to minimise mid-flexion instability by ensuring the preservation of the medial joint line. The key to achieving this is the patented Unity femoral sizer, which ensures the medial joint line is preserved in flexion regardless of the femoral rotation applied. It does this by using a built-in medial pivoting mechanism rather than the conventional central pivoting mechanism and ensures the medial posterior resection matches the implant thickness. The conjunction of the Unity femoral sizer and single radius femoral implant design ensures preservation of the medial joint line and MCL isometry throughout the range of motion.


Unity Knee enables surgeons to preserve the medial joint line, maintain mid-flexion stability and improve patient satisfaction3.


How does technology enhance this approach to joint stability?


OMNIBotics®, robotic-assisted knee technology, now supports the implantation of Unity Knee.  The BalanceBot™ provides a force-controlled, dynamic assessment of knee laxity through the entire range of motion.  Surgeons then plan the femoral resections based on this knee balance and execute the surgical plan with the OMNIBot femoral saw guide. For more information on Unity Knee and OMNIBotics visit our solutions page.




  1. Bourne RB, Chesworth BM, Davis AM, Mahomed NN, Charron KD. Patient satisfaction after total knee arthroplasty: who is satisfied and who is not? Clin Orthop Relat Res. 2010 Jan;468(1):57-63.
  2. Luyckx, Thomas; Vandenneucker, Hilde; Ing, Lennart Scheys; Vereecke, Evie; Ing, Arnout Victor; Victor, Jan, Raising the Joint Line in TKA is Associated with Mid-flexion Laxity: A Study in Cadaver Knees, Clinical Orthopaedics and Related Research: March 2018 - Volume 476 - Issue 3 - p 601-611.
  3. Van Onsem S, Knee Kinematics Determine Patient Satisfaction After TKA, The objective substrates of patient satisfaction after total knee arthroplasty, Chapter 3, 2018, Ghent University.